Evaporative Emissions System: Description and Operation

PURPOSE
The basic evaporative emission (EVAP) control system used on all vehicles is the charcoal canister storage method. Gasoline vapors from the fuel tank flow into the canister through the inlet labeled "TANK." These vapors are absorbed into the activated carbon (charcoal) storage device (canister) in order to hold the vapors when the vehicle is not operating. The canister is purged by Powertrain Control Module (PCM) control when the engine coolant temperature is over 60°C (140°F), the Intake Air Temperature (IAT) reading is over 10°C (50°F), and the engine has been running. Air is drawn into the canister through the air inlet grid. The air mixes with the vapor and the mixture is drawn into the intake manifold.

OPERATION
The EVAP canister purge is controlled by a solenoid valve that allows the manifold vacuum to purge the canister. The powertrain control module (PCM) supplies a ground to energize the solenoid valve (purge on). The EVAP purge solenoid control is pulse-width modulated (PWM) (turned on and off several times a second). The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the PCM. The output is commanded when the appropriate conditions have been met. These conditions are:
- The engine is fully warmed up.
- The engine has been running for a specified time.
- The IAT reading is above 10°C (50°F).

A continuous purge condition with no purge commanded by the PCM will set a DTC P1441.

Poor idle, stalling and poor driveability can be caused by:
- A malfunctioning purge solenoid.
- A damaged canister.
- Hoses that are split, cracked, or not connected properly.

A charcoal-filled canister captures and stores gasoline fumes. When the PCM determines that the time is right, it opens a purge valve which allows engine vacuum to draw the fumes into the intake manifold.




(1) EVAP Purge Solenoid.
(2) Throttle Body.
(3) Fuel Filler Neck.
(4) Fuel Tank.
(5) Rollover Valve.
(6) EVAP Canister.
(7) Ignition Feed.
(8) EVAP Purge Solenoid Driver Signal from PCM.




EVAP Components
EVAP Canister Purge Solenoid. Normally closed, the purge solenoid opens upon the PCM's signal to allow engine vacuum to purge gasoline fumes from the canister. Mounted on top of the upper intake manifold assembly. The PCM supplies a ground to energize the purge solenoid (purge "ON"). The EVAP purge control is pulse width modulated (PWM), and is turned "ON" and "OFF," several times a second. The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the PCM and the output is commanded when the appropriate conditions have been met.

Purge/Vacuum Hoses. Made of rubber compounds, these hoses route the gasoline fumes from their sources to the canister and from the canister to the intake air flow.




EVAP Canister. Mounted on a bracket ahead of the fuel tank, the canister stores fuel vapors until the PCM determines that engine conditions are right for them to be removed and burned.

Fuel Tank. The tank has a built-in air space designed for the collection of gasoline fumes.

Vacuum Source. The vacuum source is split between two ports, one on either side of the throttle body.